This invention relates to a motor-driven power steering apparatus, and more particularly to an improvement of a driver's steering feeling.
A motor-driven power steering apparatus has been disclosed, for instance, by Postexamined Japanese Patent Publication 3-42235/(1991). Its arrangement is as shown in FIG. 6.
In FIG. 6, reference numeral 1 designates an assist direction motor current indicating section. The assist direction motor current indicating section 1 comprises: a torque sensor 11 which detects the torsion torque of a steering shaft, and outputs a torque signal in proportion to the torsion torque as shown in FIG. 7; and an output torque value instruction function section 12 which, in response to the torque signal of the torque sensor 11, provides an assist direction motor current instruction value i according to a function as shown in FIG. 8.
Further in FIG. 6, reference numeral 2 designates a phase compensation section which outputs a signal proportional to the differential value of the torque signal of the torque sensor 11, the output signal of the section being added to the aforementioned torque signal; and 21, a phase compensation instruction function section which outputs a phase compensation current instruction value ip which, as shown in FIG. 9, changes continuously when the signal of the phase compensation section 2 is lower than a predetermined value, and is constant when the output signal of the phase compensation section is higher than the predetermined value. The phase compensation current instruction value ip is added to the assist direction motor current instruction value i of the output torque value indicating function section 21 to obtain an addition value. The addition value thus obtained is applied to a control circuit 3, to drive an electric motor 4, to power-assist a steering mechanism 5.
The aforementioned control circuit 3 comprises: a positive or negative discriminating section 31 which determines whether the addition value of the assist direction motor current instruction value i and the phase compensation current instruction value ip is positive or whether the addition value is negative; an absolute value conversion section 33 which obtains the absolute value .vertline.i+ip.vertline. of the above-described addition value; a motor drive section 32 which determines the direction of rotation of the motor 4 according to the output signal of the positive or negative value discriminating section 31 and drives the motor 4 in response to the current instruction value provided by the absolution value conversion section 33; and a duty control section 34 which compares a motor armature current detected by an armature current detecting section 35 with the current instruction value of the absolute value conversion section 33, to perform duty control.
The conventional motor-driven power steering apparatus operates as follows:
The phase compensation section 2 obtains a signal proportional to the differential value of the torque signal of the steering shaft. The assist direction motor current instruction value based on the addition signal of the aforementioned signal and the torque signal is added to the phase compensation current instruction signal which changes continuously when the output signal of the phase compensation section 2 is lower than the predetermined value, and is constant when it is higher than the predetermined value, to obtain the addition value. The addition value thus obtained is utilized to control the control section 3. This improves the motor power assistance follow-up characteristic (responsibility), and prevents the occurrence of self-oscillation at the time of rest steering, and absorbs the effect of the motor's inertial force when the vehicle is traveling at high speed, thus eliminating a sense of incongruity during steering.
In the above-described conventional motor-driven power steering apparatus, the motor 4 is driven according to the output of the torque sensor. Therefore, when the torsion torque is not applied, the motor 4 is not driven. Thereafter, when a steering operation is started, a torque loss occurs which attributes to the static friction of the motor and a steering mechanism, and therefore a great steering torque is necessary for the steering operation; that is, the driver feels unpleasant about the steering operation. In addition, when the steering wheel is released after the steering operation, the return of the steering wheel is obstructed by the friction of the motor and the steering mechanism; that is, it is rather difficult for the steering wheel to return to the neutral position (the straight movement of the vehicle).